minor #12853 [4.4][Security] Password migration with guards (wouterj)

This PR was merged into the 4.4 branch.

Discussion
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[4.4][Security] Password migration with guards

Fixes #12756

When using password migration with guard, you also need to implement an interface in your guard class.

Thanks @chalasr for reporting this missing bit.

Commits
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ea43e0d Document PasswordAuthenticatedInterface

Author: xabbuh

security/password_migration.rst

@@ -119,14 +119,41 @@ Upgrade the Password
 
 Upon successful login, the Security system checks whether a better algorithm
 is available to hash the user's password. If it is, it'll hash the correct
-password using the new hash. You can enable this behavior by implementing how
-this newly hashed password should be stored:
+password using the new hash. If you use a Guard authenticator, you first need to
+`provide the original password to the Security system <Provide the Password when using Guards>`_.
+
+You can enable the upgrade behavior by implementing how this newly hashed
+password should be stored:
 
 * `When using Doctrine's entity user provider <Upgrade the Password when using Doctrine>`_ 
 * `When using a custom user provider <Upgrade the Password when using a custom User Provider>`_ 
 
 After this, you're done and passwords are always hashed as secure as possible!
 
+Provide the Password when using Guard
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+When you're using a custom :doc:`guard authenticator </security/guard_authentication>`,
+you need to implement :class:`Symfony\\Component\\Security\\Guard\\PasswordAuthenticatedInterface`.
+This interface defines a ``getPassword()`` method that returns the password
+for this login request. This password is used in the migration process::
+
+    // src/Security/CustomAuthenticator.php
+    namespace App\Security;
+
+    use Symfony\Component\Security\Guard\PasswordAuthenticatedInterface;
+    // ...
+
+    class CustomAuthenticator extends AbstractGuardAuthenticator implements PasswordAuthenticatedInterface
+    {
+        // ...
+
+        public function getPassword($credentials): ?string
+        {
+            return $credentials['password'];
+        }
+    }
+
 Upgrade the Password when using Doctrine
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~